The present invention is directed generally to optical fiber for telecommunications and more specifically to an optical fiber capable of multimode operation at wavelengths below 1300 nm and single mode operation at wavelengths above 1300 nm, the optical fiber having reduced intermodal noise.
The optical fiber typically used to wire homes and small businesses has undesirably low bandwidth. Currently, 850 nm multimode fiber is the preferred fiber for wiring homes and small businesses because the various system components (e.g. lasers, receivers) used in conjunction with this fiber are inexpensive. However, conventional 850 nm multimode fiber can support a relatively low bit rate.
Typically, upgrading to a higher bandwidth system requires replacing the existing fiber. Conventional 850 nm multimode fiber is incompatible with higher bit rate components, such as 1300 nm single mode lasers and receivers. Therefore, it is not possible to upgrade a conventional 850 nm multimode system to a higher bit rate system without replacing all of the components, especially the fiber. Typically, to upgrade a conventional 850 nm multimode system to a higher bit rate system, the conventional 850 nm multimode fiber is replaced with a 1300 nm single mode fiber such as Corning SMF28(trademark).
Replacing the existing fiber can be expensive. For example, replacing the 850 nm multimode fiber often entails digging up the old fiber and laying the new fiber in its place. Additionally, fiber replacement often requires significant reconstruction of the home or office installation. Thus, fiber replacement is often a costly and time-consuming process.
Rather than replacing old fiber and laying new fiber, it would be preferable to initially install a fiber capable of multimode operation at 850 nm and single mode operation at 1300 nm. Thus, upgrading a system with such a fiber would only require replacing system components. Experimental fibers capable of both multimode operation at 850 nm and single mode operation at 1300 nm have been reported in the literature, however, those fibers were step indexed and tended to have very low bandwidth at 850 nm. Therefore, it would be advantageous to have a fiber capable of multimode operation at 850 nm with a large bandwidth and single mode operation at 1300 nm.
An additional problem associated with multimode fibers is intermodal noise. Intermodal noise is related to a variation of the optical intensity at a given optical fiber output location due to optical interference between modes of different phase. Many factors may act singly or in combination to produce phase changes that can cause intermodal noise. Example factors include, changes in temperature, mechanical distortions (including movement or vibration), as well as changes in optical source wavelength.
Intermodal noise is a common problem in multimode fibers used with highly coherent light sources, e.g., lasers. This is because the relative coherence of the modes allows the modes to effect the intensity of the light by interfering with each other. Less coherent sources, such as LED""s, have a short coherence length and therefore are only subject to intermodal noise in very short lengths of fiber. However, LED sources are polychromatic which causes significant pulse broadening in the fiber. This is a problem because pulse broadening reduces bandwidth. Therefore, it would be advantageous to have a fiber designed for operation with coherent light sources which does not suffer from intermodal noise.
The present invention provides an optical fiber, comprising a core having an alpha profile with an alpha parameter in a range from approximately 2 to approximately 8, a maximum index percent difference between the core and a cladding in a range from approximately 0.3% to approximately 0.5% and a core diameter in a range from approximately 6.0 to approximately 16.0 xcexcm and a cladding, wherein the optical fiber has a bandwidth of at least approximately 0.6 GHz.km at 850 nm, and is configured for multimode operation at a wavelength less than 1300 nm. Cut off wavelength of the optical fiber is in the range from about 1050 nm to 1300 nm so that single mode operation is exhibited at a wavelength of at least approximately 1300 nm.
In embodiments in accord with the invention the core diameter has a range of approximately 6.0 to 14.0 xcexcm or a maximum index percent difference in the range of approximately 0.3% to 0.4%. A preferred range of alpha parameter is from approximately 2 to approximately 4.
In an embodiment of the optical fiber in accord with the invention effective area is greater than 70 xcexcm2 at 1550 nm and preferably greater than 90 xcexcm2. The pin array bend loss is less than 4 dB at 1550 nm and preferably less than 2 dB.
In yet another embodiment of the optical fiber in accord with the invention the mode field diameter is greater than or equal to 10 xcexcm.
In a preferred embodiment of the invention, the optical fiber comprises a core and a cladding, wherein the optical fiber is a multimode fiber at an operating wavelength and is configured in accord with a given operating wavelength, the desired bandwidth, and the length of the fiber. In particular the peak bandwidth wavelength of the fiber is offset from the operating wavelength by an amount sufficient to reduce modal noise. The preferred amount of the offset depends upon fiber length, desired bandwidth, and operating wavelength.
In yet another preferred embodiment, the alpha parameter is approximately 2, the maximum index percent difference between core and clad has a range from approximately 0.35% to 0.40%, and the core diameter is in the range from approximately 14.0 to 16.0 xcexcm. The embodiment provides an optical fiber having, at 1550 nm, an effective area greater than 90 xcexcm2 and a mode field diameter greater than 11 xcexcm. Pin array bend loss is less than 2 dB at 1550 nm.
In an additional preferred embodiment, the alpha parameter is approximately 3, the maximum index percent difference between the core and the cladding is in the range of approximately 0.35% to approximately 0.4% and the core diameter is in the range of approximately 12.0 to approximately 15.0 xcexcm, to provide a waveguide fiber having effective area greater than 85 xcexcm2, and mode field diameter greater than 10.5 xcexcm. The pin array bend loss is less than 4 dB at 1550 nm.
In an additional preferred embodiment, the alpha parameter is approximately 4, the maximum index percent difference between the core and the cladding is in the range from approximately 0.3% to approximately 0.4% and the core diameter is in the range from approximately 12.0 to approximately 16.0 xcexcm, to provide a waveguide fiber having, at 1550 nm, an effective area greater than 85 xcexcm2, and mode field diameter greater than 10.5 xcexcm. The pin array bend loss at 1550 nm is less than 3.5 dB.
In an additional embodiment of the invention, the offset between peak bandwidth wavelength and operating wavelength is selected to provide respective group time delays for modes which are either all positive or all negative. The sign of the delay is determined with reference to the arrival time of the lowest order mode (LP01 mode). A positive group time delay pertains to a mode which arrives before the LP01 mode and a negative arrival time is the converse.
In another embodiment of the optical fiber in accord with the invention the absolute value of the sum of the respective group time delays is greater than 0.
A second aspect of the invention is a method of designing an optical fiber having a bandwidth of at least 0.6 GHz.km at 850 nm in multimode operation and being in single mode operation at a wavelength of at least approximately 1300 nm, comprising, determining for a given length of optical fiber a minimum difference between the operating wavelength and a peak bandwidth wavelength such that the difference in the optical path lengths of the modes in multimode operation is greater than at least one coherence length of the fiber, the coherence length being associated with a source utilized to launch light within the optical fiber at the operating wavelength, and determining an index profile associated with the optical fiber in accordance with the minimum difference.
In an embodiment of the method of the second aspect of the invention, determining the minimum difference in offset between peak bandwidth wavelength and operating wavelength includes the step of calculating a speckle constant gamma, xcex3. The speckle constant is calculated as a function of bandwidth, line width of the light source, intensity of the light source, and length of the optical fiber.
In another embodiment in accord with the method, the step of determining a minimum difference includes having the respective group delay times of the modes be all negative or all positive.
In another embodiment in accord with the method, the step of determining an index profile includes at least one of determining the operating wavelength, the desired bandwidth, or the length of the optical fiber during operation.
Another aspect of the optical fiber of the present invention includes an optical fiber system comprising an optical fiber with a core having an alpha profile with an alpha parameter from approximately 2 to approximately 8, a maximum index percent difference between the core and a cladding from approximately 0.3% to approximately 0.5% and a core diameter of approximately 6.0 to approximately 16.0 xcexcm and a light source optically coupled to the optical fiber. The alpha parameter, the maximum percent index difference, and the core diameter are chosen to provide an offset between peak bandwidth wavelength and operating wavelength sufficient to reduce intermodal noise at the operating wavelength.
An additional aspect of the invention is a method of operating an optical fiber system comprising providing an optical fiber wherein the optical fiber is a multimode fiber at an operating wavelength and has a peak bandwidth wavelength offset from the operating wavelength and operating a light source at the operating wavelength.